Non-aqueous electrolyte lithium secondary batteries have been put to practical use as secondary batteries for portable electronic equipments and are widely used. However, LiCoO2 used as the positive electrode active material is a compound of Li and Co which is less in the deposit amount as resources and are expensive, and there is demanded a positive electrode active material for non-aqueous electrolyte secondary batteries which comprises a compound containing mainly those elements which are abundant in resources.
Under the circumstances, NaNiO2 which is a composite oxide of sodium and nickel which are abundant in resources has been proposed as a positive electrode active material for non-aqueous electrolyte secondary batteries (cf. Non-patent Document 1).
However, non-aqueous electrolyte secondary batteries using NaNiO2 as a positive electrode active material suffer from the problem that the operating voltage is low, namely, about 2.0 V (cf. Patent Document 1).
Non-Patent Document 1: Solid State Ionics, Elsevier Science, 2000, Vol. 132, p. 131-141
Patent Document 1: JP-A-2003-151549